Method of cleaning up residual gases



Feb. 14, 1928.

A. E. LYLE METHOD OF CLEANING UP RESIDUAL GASES Filed NOV. 7, 192,4

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ANDREW masT INVENTOR I ATTORNEY Iatented Feb. 14, 1928. r 1,659,207UNITED STATES PATENT OFFICE.-

ANDREW ERNEST LYLE, OF HAMILTON, ONTARIO, CANADA, ASSIGNOR TO WESTING-HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA- METHOD OF CLEANING UPRESIDUAL GASES.

Application filed November 7, 1924. Serial No. 748,322.

This invention relates to a vacuum electric device and more particularlyto a method of evacuating such a device, cleaning-up the residual gasescontained therein and activating the electron-emitting element thereof.

This invention is especially adapted to the production of high vacuumsin radio tubes and similar apparatus employing an oxide coated filament.It is not limited to this 1 type of device, however, but may be appliedfor the cleaning-up of residual gases in devices employing other formsof filaments, such as pure tungsten or thoriated tungsten filaments.

uniform results may be obtained and in order to prevent arcing betweenthe electrodes and consequent destructionof the filament, that a highvacuum be produced and maintained therein. Some gases, moreover, such ascarbon dioxide and oxygen act to destroy the emissivity of the filamentand have other detrimental effects. The gases usually present in thetube after it has been exhausted to a moderately high vacuum are watervapor and carbon dioxide given olf by the glass and by the decompositionof the coating on the filament, residual air, oii vapor from the pumps,and carbon monoxide and hydrogen given off by the plate.

It is not practical to remove completely these various gases and vaporsby means of pumps since such operation is very long and expensive andthe pumps suitable for this a purpose are very costly.

It is the usual practice, therefore, in the manufacture of radio tubes,to exhaust the devices to a moderately high vacuum by means of rotaryoil pumps and to clean-up 0 the residual gases and vapors by metallicclean-up agents such as magnesium, aluminum, misch metal, etc.

In application Serial No. 636,510 of Ralph E. Myers, filed May 3, 1923,and entitled Evacuated devices and method ofexhaust,

is disclosed the use of magnesiumas a cleanup material. The use of mischmetal as a clean-up material shown and describedin application SerialNo. 679,489, Marden et al., filed December 8, .1923, and entitled.

Clean-up and activation by misch metal and the rare earth metals, and inapplication Serial No. 586,261, Rentschler, filed August 16, 1922,entitled Vacuum devices and methods of exhausting the same, there isdisclosed the use of aluminum as a. clean-up agent. All of these casesare assigned to the Westinghouse Lamp Company.

Briefly, the method employed in using these clean-up materials, is toplace a small quantity of such metal on the plate or other metal part ofthe device before the same is sealed into the bulb and after. exhaustingand sealing-oft the tube, vaporizing such material .by heating the sameinductively by ,65 high frequency currents or otherwise to render it ina condition in which it will react with the gases to produce solidcompounds thereof. Inradio tubes it is desirable, in order that 1 Whileexcellent results are obtained with the use of a metal getter alone, Ihave found that higher emissivity and greater life may be obtained bycombining therewith, a basic gettermaterial such as calcium oxide. Thisis particularly, true in the manufacture of radio tubes employing oxidecoated filaments.

In the production of oxide coated filaments, it is desirable to applythe oxides, which may be a combination of barium and strontium oxides,to the filament in the form of a coating ofthe carbonates of thesesubstances and subsequently decompose the carbonate to produce the oxidein accordance with the method disclosed in application Serial No.587,270, MacRae, filed September 11, 1922, entitled Electron-emittingdevices and method of making, and assigned to the Westinghouse LampCompany.

The use of the carbonates of barium and strontium as the coatingmaterial is extreme ly advantageous since this material may be readilyapplied to the filament and baked thereon by the ordinary equipment inuse in coating incandescent lamp filaments and other wires. Moreover, ondecomposition, the character of the oxide coating obtained, is such asto give the filament higher emission than when other known means ofproducing oxide coated filaments are used. This appears to be due to thepprous and spongy character of the oxide coating produced in this way.In decomposing carbonates of such a coating from the the oxides, a largequantity of carbon dioxide is liberated which must be eliminated fromthe device in some manner. Heretofore, in order to render possibletheuse of filaments coated with carbonates of barium and strontium and toeliminate from 0 exhausting of the device and while it is still on thepumps. This procedure, however, re-

, quires the use of a special type of very low premure mere pumpcombined with a complicated electrode treatment during the exhaustoperation. The operation moreover, is relatively slow and expensive andf the results obtained are not as uniform as I with the presentinvention.

One of the objects of this invention is to provide a method of obtaininghigh vacuums in devices containing oxide coated filaments withoutnecessitating the use of extremely low pressure pumps and which isadaptable for factory operation on rotary oil vacuum pumps.

Another object is to provide a method of obtaining high vacuums in tubesemploying oxide coated filaments which will permit the coating to beapplied to the filament in-the form of a carbonate of the coatingmaterials and to be decomposed to the oxide subsequent to the exhaustoperation.

Another object is to provide a method of evacuating an electron emissiondevice by which the life of the device and the emission of theelectron-emitting element will be increased.

Anotherobject is to provide a methodof evacuating radio tubes which willbe effective to decrease, the plate resistance thereof.

Another object is to provide a method of cleaning-up residual gases in avacuum device which will be efi'ective to maintain said vacuumthroughout the life of the device.

Other objects and advantages will hereinafter appear.

In accordance with this invention, radio tubes or other devices areevacuated to a moderately high vacuum by any suitable means such asrotary oil vacuum pumps and the residual gases retained therein arecleaned up by a metallic getter such as magnesium in combination with anon-metallic or basic material such as calcium oxide. The calcium oxideserves to clean-up the gases capable of reacting with it to form solidcompounds such as carbon dioxide and water vapor and the metallicclean-up agent removes the hydrogen, oxygen and carbon monoxide from thetube and'also assists in cleaning up the carbon dioxide and water vapor.It is believed that the calcium oxide also-occludes or absorbs the argongas present but I do not desire to restrict myself to such action sincethe same is not fully understood. a

This invention is not limited to calcium oxide as a' basic clean-upmaterial, since other alkali oxides or alkaline earth oxides may be usedfor this purpose, if desired.

The magnesium or other metal clean-up -material is introduced.

' tube is then 'tained in the tube or given off into the devicev byattaching it to one of the metal parts of the tube, preferably the platein the case of radio tubes and the calcium oxide is preferablyintroduced in a water suspension, painted or otherwise coated on thepart of the tube. After the device has been evacuated and tipped-off thebarium and strontium carbo nates on the filament are decomposed to theoxides, in a manner which will hereinafter appear, with a liberation oflarge quantities of carbon dioxide which is, partially or entirelycleaned up by the action of the calcium oxide. The plate and other metalparts are then heated by high frequency induction to vaporize themagnesium secured thereto and to liberate the gases occluded in suchparts. The volatilized magnesium will clean up those gases, such ashydrogen, oxygen, carbon monoxide, etc., not eliminated by the lime andthe excess magnesium forms a coating on the surface of the bulb. The

heating the filament with voltage on the plate and grid so as to ionizeany by the filament and render them active so that they may be readilyabsorbed by or combined With the magnesium orlime. In order that thisinvention may be more y understood, reference may be had to theaccompanying drawing in which the single figure is a side view partly insection, of a radio tube adapted to be treated in accordance with thisinvention.

The radio tube comprises a bulb or envelope 1, a stem 2 sealed therein,a filament 3, grid 4 and plate 5. The filament, grid and plate, arecarried by the stem 2 in the usual manner of constucting radio tubes andsuitable leading-in wires 6, 7 8 and 9 sealed through'the stem arejoined to the electrodes in the ordinary manner. The filament 3ispreferably of the oxide coated type and may be prepared by firstcoating a'platinum-iridium ribbon or wire of the desired size with amixture of barium andstrontium carbonates prepared by making an aqueoussuspension of a mixture of. pure powdered barium carbonate in purepowdered strontium carbonate in approximately the proportions of 9 gramsof the former with 10 grams'Of the latter and about 18 c. c. of wateradded gradually to form a suspension of proper con sistency for coating.This suspension may be applied to the wire in any suitable manner andbaked thereon in an electric tube furnace in an atmosphere of carbondioxide to prevent decomposition of the carbonates.

The plate 5 has secured thereto, before stem or other glass 7 given aseasoning treatment by gases con being sealed into the bulb, a smallpiece of 7' magnesium 10 or its equivalent metal cleanup material. Thismaterial may be attached by welding, soldering, pasting, crimping orotherwise. The amount of magnesium used a milligram.

may be very small weighing not more than With even this small amount ofmagnesium there will be an excess thereof in the bulb which, aftervaporization, will form a coating on the inside surface of the bulb andserve throughout the life of the device to maintain the vacuum therein.

The stem 2 of the tube has applied thereto,

[a coating 11 of calcium hydroxide applied thereto in any suitablemanner prior to the sealing-in operation. Calcium hydroxide ispreferably applied in the form of a water suspension produced by slakinga quantity of calcium oxide or lime with a small amount of water in a,mortar, breaking up the calsuitable g cium oxide with a pestle, addingadditional water and grinding to a paste-like consistency. The calciumhydroxide so formed, may be plainted or 0t erwise coated on a ass partof the device such as'the stem. It is preferable to use the mixturefairly soon after it is made up within from say 10 to 20 hours, as itabsorbs carbon dioxide from the atmosphere. For the same reason, it isadvisable to exhaust the device reasonably soon after the stem has beenscaled therein. If desired, the calcium oxide may be applied directly tothe stem in the form of an alcoholic suspension; however, the formermethod is preferable.

In the sealing-in and exhaust operation, the bulbs are baked at afairlyhigh temperature and the metal parts heated by high frequency inductionto drive off any gases occluded in the glass or metal parts thereof andduring this baking operation, it is believed that the calcium hydroxideis partially or entirely converted to calcium oxide. It is preferable,however, dehydrate the material since a slight amount of moisture willrender the same more active.

After sealing in the stem and exhausting the bulb to as high a vacuum asis practica in factory production, the carbonate coating on the filamentis decomposed to the oxide with the liberation of carbon dioxide andwater vapor. The decomposition of the coating of the filamentmay beaccomplished by passing a current of sufiicient strength through thefilament to heat it to the decomposing temperature of the carbonatesthereon. I have found that a current of .4 amperes through the filamentfor a period of about 30 seconds will decompose substantially all of thecarbonates.

The calcium oxide on the stem will absorb a large part of the carbondioxide and water vapor given off by the filament, the carbon dioxidecombining with the lime to form calcium carbonate and the water vapor toform calcium hydroxide. The lime may act not to completely pumps andrenders any carbon dioxide and water vapors not.

combined with the lime, the magnesium is vaporized from the platepreferably by heat ing' the same inductively by high frequency current.The vaporized magnesium reacts with substantially all of the residualgases within the tube except argon, to form solid compounds thereof andforms a deposit on the bulb which serves throughout the life of the tubeto absorb any gases formed therein.

After the vaporization of the magnesium, the tube is seasoned for about5 minutes at approximately .3 amperes through the filament and about 60volts across the plate and filament. j A potential of about 1.5 volts isalso impressed on the grid. The gases driven off during this period areionized and rendered active so that they will react with the magnesiumon the bulb and form solid compounds. If desired, the magnesium may bevaporized before the decomposition of the coating on the filament.

It will be noted that by using a combination of a metallic clean-upagent and a basic clean-up agent, that barium and strontium maybe'applied to the filament in the form of carbonates and after tippingoff the device, the carbonates decomposed into the oxides, thuseliminating the expensive and troublesome operation of decomposing thecoating on the pumps. Moreover, the tube need not be evacuated by thepumps to as high a vacuum as when no clean-up material is used-0r whenlime alone is used, since the magnesium is very efiectivefor cleaning-uplarge quantities of gas. The process, therefore, permits the use ofrotary oil vacuum the" same commercially practical.

While the exact manner in which the lime acts in the tube is not clearlyunderstood aside from its clean-up effect on the carbon dioxide 'andwater vapor, I have llll) found that a much better emission and longerlife is obtained when using this material in combination with magnesiumor other metal getter than when either of these materials are usedseparately.

I have found that by using lime in com bination with the magnesium, anincrease in emissivity of the filament of substantially 100% over theuse of magnesium alone, may be obtained. I have also found that thiscombination of getters decreases to a very large extentthe plateresistance of the tube and increased considerably the life of thedevice.

In order that the improved results obtained by using the combination oflime and magnesium getters, may be more fully appreciated, the followingtabulation of actual Tube Emission 12 average of 9 tests without 1ime.12 average of 8 tests with lime -a- 11 average of 14 tests without lime11 average of 15 tests with lime It is thought that the improved resultsare obtained because of the action of lime in absorbing those gaseswhich destroy emission and because of the liberation from the lime ofhydrogen, which in minute quantities in a free condition in the tube,appears to have a beneficial action. This hydrogen may be liberated asfree hydrogen or in the form of water vapor which is decomposed by themagnesium, the oxygen being absorbed thereby and the hydro en liberated.

I do not desire to be limited ticular theory of operation of materialsdescribed herein, or to the exact materials for and method of carryingthe invention into effect, but desire it to be understood that manychanges may be made in the process without departing from the invention.

lVhat is claimed is: i

1. The method of cleaning up residual gases and Vapors 'in a partiallyevacuated envelope containing an electrode coated with oxides of thealkaline earth metals comprising. absorbing said gases and vapors by thecombined action of a vaporizable metal and an alkalin earth oxide.

2. The method of cleaning up residual gases and vapors in a partiallyevacuated envelope containing an electrode coated with oxides ofthealkaline earth metals comprising absorbing said gases and Vapors bythe combined action of a metallic clean-up agent and line. 1

The method of producing a high vacu um in a discharge device employingan electrode coated with the oxides of alkaline earth metals whichcomprises evacuating said device to a commercially obtainable vacuum bya suitable pump and cleaning up the residual gases and vapors by thecombined action of magnesium and calcium oxide.

4. The steps in the method of producing anelectron discharge devicewhich comprise to any par the clean-up coating the filament of saiddevice with an al line earth and absorbing the residual gaswithin saiddevice and the gas liberated by said decomposition by the combinedaction of calcium oxide and a vaporizable metal.

5. The steps in the method of producing an electron discharge devicewhich com rise.

of sald devlce wlt an coating the filament alkaline earth carbonate,evacuating said device, sealing t-he same, decomposing the carbonateto-produce an alkaline earth oxide and cleaning up'the gases containedin said device by the combined action of a vaporizable metal and calciumoxide.

6. The method of activating an electronemitting element having analkaline earth carbonate applied thereto which consists in heating saidelement in an evacuated con- I tainer and absorbing the residual gasescontained in said container and the gases and vapors liberated from theelement by a vaporizable metal and an alkaline earth oxide.

7. A dischargedevice comprising an evacuated container having therein ananode, a

cathode coated with alkaline earth oxides, a quantity of metallicclean-up material and a quantity of an alkaline earth oxide cleanupmaterial.

8. A discharge device comprising an envelope having therein an anode, acathode coated with alkaline earth oxides, a quantity of magnesium and aquantity of calcium oxide.

9. A discharge device comprising an evacuated container having a stemsealed therein, a plurality of electrodes supported by said stem, one ofsaid electrodes having a coating of alkaline earth oxides thereon, afilm of metallic clean-up material disposed over the inner surface ofthe container and a coating of an alkaline earth oxide clean-up materialon said stem.

10. A discharge device" comprising an evacuated container having a stemsealed therein, a plurality of electrodes supported by said stem, one ofsaid electrodes having a coating of alkaline earth oxides thereon, afilm of magnesium disposed over the inner surface of the container and acoating of calcium oxide on the stem.

In testimony whereof, I have hereunto subscribed my name this 29th dayof October, 1924.

ANDREW ERNEST LYLE.

